The Shed is a dramatic example of thinking outside the box

The National has opened a brand new theatre. Now it wants new audiences to match

"There are all sorts of things you don't even think about when you start building a theatre from scratch", says Ben Power, looking critically at some red neon lights above the bar. "I think these are probably the wrong way round", he adds, to no one in particular.

We are standing amid the paint fumes and drill dust of the National Theatre's new baby. If you have walked down London's South Bank in the past month, or even been near Waterloo Bridge, you can't have missed it. A bright red building with four chimneys, it has landed in the theatre's grey forecourt like a big, mischievous block of Lego. Its look was apparently inspired by Amish barns and (zeitgeist alert) Scandinavian farm buildings but it most resembles a giant's shed. So it is called The Shed. And over the next nine months it will play host to inventions and experiments from Rufus Norris, Nick Payne, the teams behind War Horse and Constellations and myriad up-and-coming theatre-makers.

Opening tonight, The Shed is a temporary replacement for the Cottesloe, the National's third space, which closed for renovation in February in the first phase of the theatre's £70m redevelopment. It will reopen as the Dorfman (in honour of the Travelex founder Lloyd Dorfman, who has donated £10m to the refit) this time next year. "Going dark" in the meantime was a never an option – especially not with the Cottesloe on such a high after The Effect, The Curious Incident… and This House. They could have hired a theatre and shipped out shows to the West End for a year, but then they realised that it would cost the same to build a temporary theatre instead. How much might that be? £1.8m – entirely taken care of, apparently, by profits from War Horse's run on Broadway.

For Power, 31, an associate director at the National, formerly of Headlong, and now in charge of programming at The Shed, it is a "once-in-a-lifetime opportunity" to build a new theatre, and a new rep, to order. The 230-seat space is designed by Haworth Tompkins, architects of the wider NT Future project, and recalls their stripped-back work at the Young Vic and the Royal Court. Their brief here was to be temporary, cheap and sustainable. The building is made of 100 per cent recyclable timber and raw steel. The chimneys allow it to heat and cool itself efficiently. The seats and lights are borrowed from the Cottesloe and there are no wings, no backstage and just one tiny dressing room with bare bulbs and plywood walls. "I have a fantasy that all of the performers who come in will scratch their name on the wall, and at the end it will be covered," says Power.

While plugged into the powerhouse of the National, sharing its facilities and talent, every effort has been made to make The Shed feel like its own theatre. It has its own entrance, its own trendily scruffy bar, its own pricing (tickets will be £12 or £20, programmes £1) and, it is hoped, its own audience. The National has been trying to seduce a younger crowd for some time – most successfully with cheap Travelex tickets but also with a season of new writing staged in its set-painting warehouse last year, and late-night summer bar, Propstore. The Shed is its most concerted effort yet to tempt audiences from London's more experimental arts spaces.

"There are people who go to see new writing Upstairs at the Royal Court or at the Arcola, or who go to the Young Vic for its bar", says Power. "And they don't come to the National because they think it's establishment and not for them or they find the building intimidating or the programming old-fashioned or over- literate. Actually that audience would really enjoy what we do. This is a way of reaching towards that."

The Shed will be a space to experiment, giving new ideas and artists a platform at the heart of the theatre establishment, but away from the glare of the main house lights. One day, they might make it inside to the Lyttelton. Much of the work has its origins in the National Theatre Studio, including the opening show Table, a multi-generational family epic directed by Rufus Norris. In the autumn, debbie tucker green, a Royal Court stalwart, will make her NT debut with Nut and Tim Price will unveil his play about the Occupy movement. At Christmas, Power will team up with Marianne Elliott and others from the War Horse team on Elephantom, a family show, and in 2014 Nick Payne (Constellations) will tackle gender relations in a new play directed by Carrie Cracknell. Newer talents will also get mini-runs at the venue. Some, like Scottish theatre maker Rob Drummond and American experimentalists The TEAM, were spotted by Power at last year's Edinburgh Festival. Other fringe favourites, like Little Bulb, will take part in a mini-festival in September.

Then in February the red slats will be torn down, the seats and lights refitted in the Dorfman and all that will be left of The Shed will be a plywood dressing room wall covered in signatures. "And that's good", says Power. "What's important is that it happens and that the energy then flows back into the bloodstream of the theatre."

The Shed opens tonight with 'Table' (020 7452 3244; theshedtheatre.co.uk)